wip - alpha support for polymorphism

An initial update to support polymorphism from SMTLIB3 and the API (so far C, Python).

The WIP SMTLIB3 format is assumed to be supporting the following declaration

```
(declare-type-var A)
```
Whenever A is used in a type signature of a function/constant or bound quantified variable, it is taken to mean that all instantiations of A are included in the signature and assertions.
For example, if the function f is declared with signature A -> A, then there is a version of f for all instances of A.
The semantics of polymorphism appears to follow previous proposals: the instances are effectively different functions.
This may clash with some other notions, such as the type signature forall 'a . 'a -> 'a would be inhabited by a unique function (the identity), while this is not enforced in this version (and hopefully never because it is more busy work).

The C API has the function 'Z3_mk_type_variable' to create a type variable and applying functions modulo polymorphic type signatures is possible.
The kind Z3_TYPE_VAR is added to sort discriminators.

This version is considered as early alpha. It passes a first rudimentary unit test involving quantified axioms, declare-fun, define-fun, and define-fun-rec.

src/ast/polymorphism_inst.cpp

@@ -0,0 +1,138 @@
+/*++
+Copyright (c) 2023 Microsoft Corporation
+
+Module Name:
+
+    polymorphism_inst.cpp
+
+Abstract:
+
+    Utilities for instantiating polymorphic assertions.
+
+Author:
+
+    Nikolaj Bjorner (nbjorner) 2023-7-8
+
+
+--*/
+#include "ast/polymorphism_inst.h"
+#include "ast/ast_pp.h"
+
+namespace polymorphism {
+    
+    void inst::add(expr* e) {
+        if (!m.has_type_vars())
+            return;
+
+        if (m_from_instantiation.contains(e))
+            return;
+
+        instances inst;
+        u.collect_poly_instances(e, inst.m_poly_fns);
+        if (inst.m_poly_fns.empty())
+            return;
+        if (m_instances.contains(e))
+            return;
+        
+        add_instantiations(e, inst.m_poly_fns);
+
+        if (!u.has_type_vars(e))
+            return;
+            // insert e into the occurs list for polymorphic roots
+            ast_mark seen;
+            for (auto* f : inst.m_poly_fns) {
+                f = m.poly_root(f);
+                if (seen.is_marked(f))
+                    continue;
+                seen.mark(f, true);
+                if (!m_occurs.contains(f)) {
+                    m_occurs.insert(f, ptr_vector<expr>());
+                    t.push(insert_map(m_occurs, f));
+                }
+                auto& es = m_occurs.find(f);
+                es.push_back(e);
+                t.push(remove_back(m_occurs, f));
+            }
+            m_assertions.push_back(e);
+            t.push(push_back_vector(m_assertions));
+            u.collect_type_vars(e, inst.m_tvs);
+            inst.m_subst = alloc(substitutions);
+            inst.m_subst->insert(alloc(substitution, m));
+            m_instances.insert(e, inst);            
+            t.push(new_obj_trail(inst.m_subst));
+            t.push(insert_map(m_instances, e));        
+    }
+
+    void inst::collect_instantiations(expr* e) {
+        ptr_vector<func_decl> instances;
+        u.collect_poly_instances(e, instances);
+        add_instantiations(e, instances);
+    }
+
+    void inst::add_instantiations(expr* e, ptr_vector<func_decl> const& instances) {
+        for (auto* f : instances) {
+            if (m_in_decl_queue.is_marked(f))
+                continue;
+            m_in_decl_queue.mark(f, true);
+            m_decl_queue.push_back(f);
+            t.push(add_decl_queue(*this));
+        }
+    }
+    
+    void inst::instantiate(vector<instantiation>& instances) {
+        unsigned num_decls = m_decl_queue.size();
+        if (m_assertions_qhead < m_assertions.size()) {
+            t.push(value_trail(m_assertions_qhead));
+            for (; m_assertions_qhead < m_assertions.size(); ++m_assertions_qhead) {
+                expr* e = m_assertions.get(m_assertions_qhead);
+                for (unsigned i = 0; i < num_decls; ++i)
+                    instantiate(m_decl_queue.get(i), e, instances);
+            }
+        }       
+        if (m_decl_qhead < num_decls) {
+            t.push(value_trail(m_decl_qhead));
+            for (; m_decl_qhead < num_decls; ++m_decl_qhead) {
+                func_decl* p = m_decl_queue.get(m_decl_qhead);
+                for (expr* e : m_occurs[m.poly_root(p)])
+                    instantiate(p, e, instances);
+            }
+        }
+    }
+
+    void inst::instantiate(func_decl* f1, expr* e, vector<instantiation>& instances) {
+        auto const& [tv, fns, substs] = m_instances[e];
+                
+        for (auto* f2 : fns) {
+            substitution sub1(m), new_sub(m);
+            if (!u.unify(f1, f2, sub1))
+                continue;
+            if (substs->contains(&sub1))
+                continue;
+            substitutions new_substs;
+            for (auto* sub2 : *substs) {
+                if (!u.unify(sub1, *sub2, new_sub))
+                    continue;
+                if (substs->contains(&new_sub))
+                    continue;
+                if (new_substs.contains(&new_sub))
+                    continue;
+                expr_ref e_inst = new_sub(e);
+                if (!m_from_instantiation.contains(e_inst)) {
+                    collect_instantiations(e_inst);
+                    auto* new_sub1 = alloc(substitution, new_sub);
+                    instances.push_back(instantiation(e, e_inst, new_sub1));
+                    new_substs.insert(new_sub1);
+                    m_from_instantiation.insert(e_inst);
+                    m.inc_ref(e_inst);
+                    t.push(insert_ref_map(m, m_from_instantiation, e_inst));
+                }
+            }
+            for (auto* sub2 : new_substs) {
+                SASSERT(!substs->contains(sub2));
+                substs->insert(sub2);
+                t.push(new_obj_trail(sub2));
+                t.push(insert_map(*substs, sub2));
+            }
+        }        
+    }
+}